Catalysts based on the transition elements find wide application in the processing of hydrocarbon feed stocks. They may be employed in treating feed stocks derived from petroleum sources as well as from other fossil fuels such as coal.
Many hydrodesulfurization and hydrotreatment catalysts contain cobalt and molybdenum deposited on an alumina carrier. Such catalysts have also been employed for methanation, production of hydrocarbons by the Fischer-Tropsch synthesis, denitrogenation, hydroforming, hydrocracking, coal liquefaction and the water gas shift reaction. In some formulations nickel and molybdenum have been used, as well as nickel and tungsten, and in some cases supports containing such materials as silica have been used. Vanadium oxide catalysts supported on various carriers have also been found to be active in promoting the above types of hydrocarbon conversions.
After having been charged into an industrial reactor the catalysts are often activated by a sulfur containing reducing atmosphere, and remain in contact with sulfur and reducing substances throughout their life. However, the various forms of lattice and surface sulfur compounds are not well understood. Usually a considerable proportion of the oxygen bound to the transition metals remains unconverted to sulfide, and it is possible that metal binding to both oxygen and sulfur may be present under working conditions.
The book Weisser and Landa, Sulfide Catalyst--Their Preparation and Application, Pergamon Press, 1973, gives details regarding the preparation of many sulfide catalysts. In addition to catalyst preparation by decomposition of solid material, catalysts of this type have been prepared by impregnation on a carrier and by precipitation. Impregnation is the technique believed most frequently employed, but has the disadvantage that some type of activation is usually needed to convert the catalyst to the desired active form. An additional disadvantage is that the high surface area of the support does not always contribute to the production of a catalyst with a high surface area of active components, since subsequent activation reactions change the nature and aggregation of the starting materials that have been deposited on the support.
The employment of sulfide catalysts, particularly in methanation and the Fischer-Tropsch synthesis, could achieve greater commercial significance if they possessed sufficiently high activity. At present nickel based catalysts are used in methanation, because of their high activity, but they have the disadvantage of extreme sensitivity to poisoning by sulfur compounds that are typically present in synthesis gas. In the case of the Fischer-Tropsch synthesis, iron catalysts are employed because they are inexpensive, though their activity and selectivity for producing hydrocarbons in the gasoline boiling range are not high. Consequently, considerable effort has been devoted to the synthesis of new catalysts such as those based on the transition metal sulfides.
Proposals which have recently received considerable attention for the preparation of molybdenum and tungsten sulfide catalysts involve the decomposition of thiosalts. The necessary thiosalts can be prepared by precipitation from ammonium salts of the acids of molybdenum and tungsten by hydrogen sulfide. The resulting thiosalts are then decomposed in the presence of hydrogen or of an inert atmosphere. Details of such proposals are given in Kurtok et al. U.S. Pat. Nos. 3,764,649 and 3,876,755 and in Naumann et al. U.S. Pat. Nos. 4,243,553 and 4,243,554. Molybdenum trisulfide is said to be an intermediate in the formation of the molybdenum disulfide catalyst prepared by this procedure, and methods have been discussed in the literature for alternative preparation starting with the trisulfide. For example, see Furimsky and Amberg, Can.J. Chem 53, 3567 (1975), for a recent survey.
Generally in preparing these catalysts it is desirable to heat the thiosalt or trisulfide to a temperature in the range of 400.degree.-500.degree. C. This essentially converts the precursors to a high surface area molybdenum disulfide MoS.sub.2, since above 335.degree. C. the trisulfide begins to decompose to the disulfide and between 350.degree. and 400.degree. C. an exotherm occurs which is thought to be due to the combined effect of the decomposition of the trisulfide, volatilization of sulfur and recrystallization of the amorphous disulfide (according to results reported by Prasad et al, J. Inorg. Nucl. Chem 35, 1895 (1973)). Thus, the procedure proposed by Naumann and others for the preparation of catalyst is said to lead to the production of molybdenum disulfide whose activity is enhanced by the high surface area material obtained (50-150 sq.m/g). The formation of MoS.sub.2 in this manner has been reported to be irreversible, and the ratio of molybdenum to sulfur is said to be very close to that required by stoichiometry. See, for example, Busetto et al., Bull. Soc. Chim. Belg. 90 1233 (1981) for recent studies.
Even when the catalyst composition corresponds to an atomic ratio of Mo/S=1/2, enhanced activity of catalysts prepared from molybdenum trisulfide has been reported due to the incorporation of hydrogen in the structure in the course of preparation. See, for example, Blake et al., Proc. 7th Int'l Congress on Catalysis, Elsevier, 1981.
Other studies reported by Chianelli, et al., Science 203, 1106 (1979) indicate considerable complexity exists in the case of MoS.sub.2 structures that can be prepared in the form of highly folded and disordered S-Mo-S layers.
It therefore appears that much remains unknown about the optimum active forms of transition metal sulfides and the optimum methods for their preparation. The activities of these materials is still reported to be much lower than that of the Group VIII metals commonly used for similar reactions.
Another important property of the transition metal sulfide catalysts insofar as CO hydrogenation is concerned is their high selectivity for CO.sub.2 formation. In the case of methanation over nickel catalysts, for example, the process proceeds mainly according to the reaction: EQU 3H.sub.2 +CO=CH.sub.4 +H.sub.2 O (1)
But in the case of transition metal sulfides, the following methanation reaction predominates: EQU 2CO+2H.sub.2 =CH.sub.4 +CO.sub.2 ( 2)
This constitutes a substantial advantage for the case of transition metal sulfide catalysts because it is not necessary, as in the case of nickel, to first produce hydrogen by the water gas shift reaction which requires large amounts of steam: EQU H.sub.2 O+CO=H.sub.2 +CO.sub.2 ( 3)
The hydrogen then is consumed with these catalysts to produce steam again, resulting in an expensive cycling of steam.
There is thus considerable incentive to find a way to enhance the activity of transition metal sulfide catalysts as compared to, for example, Group VIII metals catalyst, while retaining other, desirable properties of the transition metal sulfide catalysts.